In semiconductor processing, in particular wafer packaging, the use of polymer materials has become very important. The processing of substrates like silicon wafers coated with polyimide (PI) or polybenzobisoxazole (PBO) in vacuum coating tools has become a very common problem due to heavy outgassing of these materials. Even more problematic is the situation with the so called enhanced Wafer Level ball grid array packaging technology (eWLB), where the silicon dice are embedded in a wafer made of polymer resin, also called a fanout wafer.
As part of the wafer packaging process a metallization is performed on patterned wafer 5 such as that depicted in FIG. 1. The wafer 5 shown in FIG. 1 includes a base substrate 10 (e.g. silicon), and metal layer(s) 12 (which may be discontinuous) having exposed contact portions 13. The contact portions 13 of the metal layers are accessible through a superjacent patterened insulating layer 14 composed of organic polymer material (e.g. PI or PBO). The goal is to provide metal contact layers (or contacts) 16 and 18 (seen in FIG. 2) for the contact portions 13 of the metal layers 12. The contacts 16,18 provide a path of electrical conductivity with said layers 12 that remains accessible through the openings in the patterned insulating layer 14. The process proceeds generally in two steps; namely an etching step to remove metal oxide deposits from the contact portions of the metal layers 12, followed by a metallization step to deposit the metal contacts 16,18.
In the first step, illustrated in FIG. 1, sputter etching by a plasma of an inert gas, typically Argon ions (Ar+), is used for the removal of metal oxide (MeO) that has formed on the exposed contact portions 13 of the metal layers 12. The MeO layers 9 are illustrated schematically in FIG. 1 with exaggerated dimensions for ease of observation (except as expressly indicated, no drawing figures in this application are drawn to scale). Removal of such oxides is desirable to maximize the conductivity and adhesion between the metal layer 12 and the later-applied contact layers 16 and 18.
Unfortunately, the plasma etching is not confined to just the metal contact portions 13. Rather, the plasma also etches the organic insulation layer 14, thus accelerating the evolution of volatile compounds (VCs) therefrom and liberating polymer material, as well. These VCs and liberated polymer can contaminate the metal contact portions 13 and interfere with etching them to remove MeO. In the second step, illustrated in FIG. 2, at least one contact layer (two contact layers 16 and 18 are shown in the illustrated embodiment) is applied. The contact layers 16 and 18 can be, e.g., titanium followed by copper. These contact layers 16 and 18 are typically applied via sputtering from an appropriate metal target, e.g. Ti or Cu, not shown in the figure. It is to be appreciated that like the plasma sputtering in the first step, this metal sputtering process also is not specific to only the contact areas 13. Rather, a metal coating will be applied over the entire wafer 5 surface (including the upper surface of the polymeric insulating layer 14) as indicated schematically by the arrows in FIG. 2. This means that the metal layers for contacts 16 and 18 extend beyond the contact portions 13 as-applied, and cover the entire upper surface of the wafer 5 as illustrated in FIG. 2.
Once the metal coating(s) for the contacts 16,18 has/have been deposited the liberation of VCs and organic material from that layer 14 will cease because the metal coating deposited on top of the insulating layer 14 will act as a barrier to the transmission of VCs and other organics. Following deposition of the metal layers for contacts 16, 18, a subsequent photolithography step typically is carried out to separate portions of those layers that will form the desired conductive circuits for the contacts 16,18. The subsequent photolithography step is beyond the scope of the present disclosure.
The foregoing two steps generally are carried out successively in respective etching and sputter chambers. In the first step carried out in the etching chamber, that chamber is initially pumped down to a vacuum and a degassing operation is performed at elevated temperature. The degassing operation is important since a certain degas temperature should not be exceeded to avoid degradation of the polymer insulating layer 14. Otherwise the outgassing is a function of time which cannot be easily accelerated. Long outgassing times however are not wanted since these slow down the throughput. Following degassing, the sputter etch operation using an inert gas plasma (typically argon) is carried out to clean the exposed metal surfaces (contact portions 13) of contaminating MeO. This process is relatively simple but it has the disadvantage that it is not selective and the Ar+ ions etch not only MeO from the exposed contact-portion surface but also polymeric material from the insulating layer 14. The resulting etched polymer often forms a thick deposit of bad integrity on the chamber walls, resulting in particles. In addition, because the etch process introduces heat to the wafer there is an increasing amount of volatile compounds (VC) outgassing from the wafer—mainly water vapour and organic residues from the insulating layer 14—over the etch process time. The evolution of these VCs via outgassing can be seen as a pressure rise in the etching chamber. These VCs leads to the effect that the MeO cannot be properly cleaned from the exposed contact portions of the metal layers due to incorporation of contaminants (i.e. volatile organics). Finally, the etch clean rate of MeO from the metal surfaces is lower than the contamination rate of those surfaces based on VCs outgassed from the insulating layer 14, resulting in a bad contact resistance.
In the second step discussed above, the wafer is moved to a sputter chamber where the metal contact layers 16,18 are applied. In the case of two or more layers, the sputter-metal deposition may be applied in two or more successive sputter chambers, one for each of the applied layers. Because the metal sputtering operation deposits metal over the polymeric insulating layer 14 and the deposited metal acts as a barrier to outgassing, it is primarily outgassing during the first step discussed above (sputtering with Ar+ plasma to remove MeO from the exposed metal contact portions 13), for which problems associated with excessive polymer sputtering and outgassing must be addressed.
Thus, there is a need in the art to improve the etch process in step 1 discussed above to achieve low particle generation from etched polymer material from the insulating layer 14, and to minimize contact resistance at the interfaces between the metal layer(s) 12 and the respective contacts 16,18 as a result of excessive MeO or other contaminants liberated from the insulating layer 14 during the etching process (i.e. polymer particles and VCs).